The present invention relates to a base coat where the last and cover layer is a clear lacquer which contains a modified polyester resin as the essential film-forming agent, prepared by ensuing baking, and it further relates to a coating process for making such an enamel multi-coat. The last layer of the base coat to be covered by the clear-lacquer coat contains pigments and/or metal pigments. The clear-lacquer coating is meant to protect the overall enamel against mechanical, chemical and weather-induced stresses, and furthermore, it is meant to contribute to reinforcing esthetic effects, for instance in the case of metal-effect or luster enamel. Base coats built up from at least one layer of lacquers are used in high-grade baking enamels, for instance in motor vehicle enameling. The clear lacquer coating forming the last layer of the base coat as a rule will be baked. It is obtained from clear lacquers containing alkyd resins from phthalic acids and polyols modified with various fatty acids or other monocarboxylic acids as film-forming agents, and aminoplast resins.
The state of the art of articles having a multilayer coating and processes for producing the same may be ascertained by reference to U.S. Pat. No. 3,639,147 of Benefiel et al which issued Feb. 1, 1972, the disclosure of which is incorporated herein.
According to Benefiel et al, an article having a substrate and a multilayer coating adhered thereto is formed from a composition comprising:
A. a dried nonaqueous base film produced from a composition comprising:
1. a first film-forming material selected from the group consisting of alkyd resins, polyester resins, acrylic resins, and polyurethane resins, wherein said first film-forming material contains from about 2 to about 50 weight percent, based on the film-forming solids, of a cellulose ester, PA1 2. uniformly dispersed pigments, and PA1 3. volatile organic solvents for said first film-forming material, and PA1 1. a second film-forming material selected from the group consisting of alkyd resins, polyester resins, acrylic resins and polyurethane resins, and PA1 2. volatile organic solvents for said second film-forming material.
B. a baked transparent coating composition deposited on said base film in a relationship to maintain substantially no intermixing and no intersolution of said base film and said transparent coating composition, said transparent coating composition comprising:
In the specification of U.S. Pat. No. 3,639,147, a clear coat is applied to a basecoat containing metal pigments. Two different proposals are made for the composition of the clear coat. One refers to the use of acrylate resins in combination with aminoplast resins as binder (Column 3). The second proposal relates to alkyd resins as binders, namely those consisting of polybasic acids and polyols and which, moreover, are modified with fatty acids (Column 4 to Column 5, line 30). The polyester resins of the present application differ from the polyester resins of U.S. Pat. No. 3,639,147 in that the polyester resins may not have any aromatic groups within the molecular structure. Only when this happens, will there be no cracking of the clear coat in the case of solar radiation. The present invention can additionally contain acrylate resins, but these resins, too, whould not have aromatic groups in their structure. When there are any aromatic groups in the molecule the crack resistance of the coating is lost. The more aromatic groups the molecule contains, the more the crack resistance worsens. The upper limit, whereby aromatic groups should not be contained in the molecule of the acrylate resin amounts to about 15%.
Alkyd resins containing essentially phthalic acids as the esterification component and modified with saturated and/or unsaturated monocarboxylic acids are known together with aminoplast resins as the binder component in coating compositions which offer high-grade coatings. The aminoplast resins are preferably used in the form of melamine formaldehyde resins and the coating compositions are in the form of solutions in organic solvents. In many cases, the coating compositions also contain levelling agents, surfactants or reaction catalysts.
Other clear lacquers besides the alkyd resins also contain poly(meth)acrylates or their copolymers. Occasionally the alkyd resins may also be replaced entirely by such poly(meth)acrylates or copolymers. They are obtained by copolymerizing (meth)acrylic acid esters of monovalent alcohols with (meth)acrylic acid esters containing functional hydroxyl groups. They may also contain other comonomers such as styrene, vinyl toluol, vinyl ester and also small amounts of monomers containing carboxyl groups polymerized therein. These acrylate resins also are used in combination with aminoplast resins in organic solvents.
These known clear lacquers are characterized by good hardness, elasticity and resistance to chemical corrosion. When used as covering lacquers in enamel multi-coats in which the last layer to be covered contains metal pigments, that is, for metallic looking enamels, the known clear lacquers following exposure to weather and ultra-violet illumination--as is the case in sunny climates--will crack. Where clear lacquers made of acrylate resins are concerned, they are furthermore difficult to apply and clear coatings made of them following exposure to weather or ultraviolet light also tend to become matte, whereby the metallic effect becomes unsightly.